Mitochondria are organelles in eukaryotic cells, popularly referred to as the “powerhouse” of the cell. One of their primary functions is oxidative phosphorylation. The molecule adenosine triphosphate (ATP) functions as an energy “currency” or energy carrier in the cell, and eukaryotic cells derive the majority of their ATP from biochemical processes carried out by mitochondria. These biochemical processes include the citric acid cycle (the tricarboxylic acid cycle, or Krebs cycle), which generates reduced nicotinamide adenine dinucleotide (NADH+H+) from oxidized nicotinamide adenine dinucleotide (NAD+), and oxidative phosphorylation, during which NADH+H+ is oxidized back to NAD+. (The citric acid cycle also reduces flavin adenine dinucleotide, or FAD, to FADH2; FADH2 also participates in oxidative phosphorylation.)
The electrons released by oxidation of NADH+H+ are shuttled down a series of protein complexes (Complex I, Complex II, Complex III, and Complex IV) known as the mitochondrial respiratory chain. These complexes are embedded in the inner membrane of the mitochondrion. Complex IV, at the end of the chain, transfers the electrons to oxygen, which is reduced to water. The energy released as these electrons traverse the complexes is used to generate a proton gradient across the inner membrane of the mitochondrion, which creates an electrochemical potential across the inner membrane. Another protein complex, Complex V (which is not directly associated with Complexes I, II, III and IV) uses the energy stored by the electrochemical gradient to convert ADP into ATP.
The citric acid cycle and oxidative phosphorylation are preceded by glycolysis, in which a molecule of glucose is broken down into two molecules of pyruvate, with net generation of two molecules of ATP per molecule of glucose. The pyruvate molecules then enter the mitochondria, where they are completely oxidized to CO2 and H2O via oxidative phosphorylation (the overall process is known as aerobic respiration). The complete oxidation of the two pyruvate molecules to carbon dioxide and water yields about at least 28-29 molecules of ATP, in addition to the 2 molecules of ATP generated by transforming glucose into two pyruvate molecules. If oxygen is not available, the pyruvate molecule does not enter the mitochondria, but rather is converted to lactate, in the process of anaerobic respiration.
The overall net yield per molecule of glucose is thus approximately at least 30-31 ATP molecules. ATP is used to power, directly or indirectly, almost every other biochemical reaction in the cell. Thus, the extra (approximately) at least 28 or 29 molecules of ATP contributed by oxidative phosphorylation during aerobic respiration are critical to the proper functioning of the cell. Lack of oxygen prevents aerobic respiration and will result in eventual death of almost all aerobic organisms; a few organisms, such as yeast, are able to survive using either aerobic or anaerobic respiration.
When cells in an organism are temporarily deprived of oxygen, anaerobic respiration is utilized until oxygen again becomes available or the cell dies. The pyruvate generated during glycolysis is converted to lactate during anaerobic respiration. The buildup of lactic acid is believed to be responsible for muscle fatigue during intense periods of activity, when oxygen cannot be supplied to the muscle cells. When oxygen again becomes available, the lactate is converted back into pyruvate for use in oxidative phosphorylation.
Mitochondrial dysfunction contributes to various disease states. Some mitochondrial diseases are due to mutations or deletions in the mitochondrial genome. If a threshold proportion of mitochondria in the cell is defective, and if a threshold proportion of such cells within a tissue have defective mitochondria, symptoms of tissue or organ dysfunction can result. Practically any tissue can be affected, and a large variety of symptoms may be present, depending on the extent to which different tissues are involved.
One such disease is Friedreich's ataxia (FRDA or FA). Friedreich's ataxia is an autosomal recessive neurodegenerative and cardiodegenerative disorder caused by decreased levels of the protein frataxin. Frataxin is important for the assembly of iron-sulfur clusters in mitochondrial respiratory-chain complexes. Estimates of the prevalence of FRDA in the United States range from 1 in every 22,000-29,000 people (see www.nlm.nih.gov/medlineplus/ency/article/001411.htm) to 1 in 50,000 people (see www.umc-cares.org/health_info/ADAM/Articles/001411.asp). The disease causes the progressive loss of voluntary motor coordination (ataxia) and cardiac complications. Symptoms typically begin in childhood, and the disease progressively worsens as the patient grows older, patients eventually become wheelchair-bound due to motor disabilities.
Another disease linked to mitochondrial dysfunction is Leber's Hereditary Optic Neuropathy (LHON). The disease is characterized by blindness which occurs on average between 27 and 34 years of age; blindness can develop in both eyes simultaneously, or sequentially (one eye will develop blindness, followed by the other eye two months later on average). Other symptoms may also occur, such as cardiac abnormalities and neurological complications.
Yet another syndrome resulting from mitochondrial defects is mitochondrial myopathy, encephalopathy, lactacidosis, and stroke (MELAS). The disease can manifest itself in infants, children, or young adults. Strokes, accompanied by vomiting and seizures, are one of the most serious symptoms; it is postulated that the metabolic impairment of mitochondria in certain areas of the brain is responsible for cell death and neurological lesions, rather than the impairment of blood flow as occurs in ischemic stroke. Other severe complications, including neurological symptoms, are often present, and elevated levels of lactic acid in the blood occur.
Yet another syndrome resulting from a respiratory chain disorder is Myoclonus Epilepsy Associated with Ragged-Red Fibers (MERRF) syndrome, one of a group of rare muscular disorders that are called mitochondrial encephalomyopathies. Mitochondrial encephalomyopathies are disorders in which a defect in the genetic material arises from a part of the cell structure that releases energy (mitochondria). This can cause a dysfunction of the brain and muscles (encephalomyopathies). The mitochondrial defect as well as “ragged-red fibers” (an abnormality of tissue when viewed under a microscope) are always present. The most characteristic symptom of MERRF syndrome is myoclonic seizures that are usually sudden, brief, jerking, spasms that can affect the limbs or the entire body. Impairment of the ability to coordinate movements (ataxia), as well as an abnormal accumulation of lactic acid in the blood (lactic acidosis) may also be present in affected individuals. Difficulty speaking (dysarthria), optic atrophy, short stature, hearing loss, dementia, and involuntary jerking of the eyes (nystagmus) may also occur.
Yet another syndrome is Leigh's disease, a rare inherited neurometabolic disorder characterized by degeneration of the central nervous system. Leigh's disease can be caused by mutations in mitochondrial DNA or by deficiencies of pyruvate dehydrogenase. Symptoms of Leigh's disease usually begin between the ages of 3 months to 2 years and progress rapidly. In most children, the first signs may be poor sucking ability and loss of head control and motor skills. These symptoms may be accompanied by loss of appetite, vomiting, irritability, continuous crying, and seizures. As the disorder progresses, symptoms may also include generalized weakness, lack of muscle tone, and episodes of lactic acidosis, which can lead to impairment of respiratory and kidney function. Heart problems may also occur. In rare cases, Leigh's disease can begin during late adolescence or early adulthood and progress more slowly.
Yet another syndrome resulting from a respiratory chain disorder is Co-Enzyme Q10 Deficiency, the symptoms of which include encephalomyopathy, mental retardation, exercise intolerance, ragged-red fibers, and recurrent myoglobin in the urine.
Yet another syndrome resulting from a respiratory chain disorder is Complex I Deficiency or NADH dehydrogenase NADH-CoQ reductase deficiency, the symptoms of which are classified by three major forms: (1) fatal infantile multisystem disorder, characterized by developmental delay, muscle weakness, heart disease, congenital lactic acidosis, and respiratory failure; (2) myopathy beginning in childhood or in adult life, manifesting as exercise intolerance or weakness; and (3) mitochondrial encephalomyopathy (including MELAS), which may begin in childhood or adult life and consists of variable combinations of symptoms and signs, including ophthalmoplegia, seizures, dementia, ataxia, hearing loss, pigmentary retinopathy, sensory neuropathy, and uncontrollable movements.
Yet another syndrome resulting from a respiratory chain disorder is Complex II Deficiency or Succinate dehydrogenase deficiency, the symptoms of which include encephalomyopathy and various manifestations, including failure to thrive, developmental delay, hyoptonia, lethargy, respiratory failure, ataxia, myoclonus and lactic acidosis.
Yet another devastating syndrome resulting from a respiratory chain disorder is Complex II Deficiency or Ubiquinone-cytochrome C oxidoreductase deficiency, symptoms of which are categorized in four major forms: (1) fatal infantile encephalomyopathy, congenital lactic acidosis, hypotonia, dystrophic posturing, seizures, and coma; (2) encephalomyopathies of later onset (childhood to adult life): various combinations of weakness, short stature, ataxia, dementia, hearing loss, sensory neuropathy, pigmentary retinopathy, and pyramidal signs; (3) myopathy, with exercise intolerance evolving into fixed weakness; and (4) infantile histiocytoid cardiomyopathy.
Yet another syndrome resulting from a respiratory chain disorder is Complex IV Deficiency or cytochrome C oxidase deficiency, caused by a defect in Complex IV of the respiratory chain, the symptoms of which can be categorized in two major forms: (1) encephalomyopathy, which is typically normal for the first 6 to 12 months of life and then show developmental regression, ataxia, lactic acidosis, optic atrophy, ophthalmoplegia, nystagmus, dystonia, pyramidal signs, respiratory problems and frequent seizures; and (2) myopathy: Two main variants: (a) Fatal infantile myopathy: may begin soon after birth and accompanied by hypotonia, weakness, lactic acidosis, ragged-red fibers, respiratory failure, and kidney problems: and (b) Benign infantile myopathy: may begin soon after birth and accompanied by hypotonia, weakness, lactic acidosis, ragged-red fibers, respiratory problems, but (if the child survives) followed by spontaneous improvement.
Yet another syndrome resulting from a respiratory chain disorder is Complex V Deficiency or ATP synthase deficiency includes symptoms such as slow, progressive myopathy.
Yet another syndrome resulting from a respiratory chain disorder is CPEO or Chronic Progressive External Ophthalmoplegia Syndrome includes symptoms such as visual myopathy, retinitis pigmentosa, or dysfunction of the central nervous system.
Another mitochondrial disease is Kearns-Sayre Syndrome (KSS). KSS is characterized by a triad of features including: (1) typical onset in persons younger than age 20 years; (2) chronic, progressive, external ophthalmoplegia; and (3) pigmentary degeneration of the retina. In addition. KSS may include cardiac conduction defects, cerebellar ataxia, and raised cerebrospinal fluid (CSF) protein levels (e.g., >100 mg/dL). Additional features associated with KSS may include myopathy, dystonia, endocrine abnormalities (e.g., diabetes, growth retardation or short stature, and hypoparathyroidism), bilateral sensorineural deafness, dementia, cataracts, and proximal renal tubular acidosis. Thus, KSS may affect many organ systems.
In addition to congenital disorders involving inherited defective mitochondria, acquired mitochondrial dysfunction contributes to diseases, particularly neurodegenerative disorders associated with aging like Parkinson's, Alzheimer's, and Huntington's Diseases. The incidence of somatic mutations in mitochondrial DNA rises exponentially with age; diminished respiratory chain activity is found universally in aging people. Mitochondrial dysfunction is also implicated in excitoxic, neuronal injury, such as that associated with cerebral vascular accidents, seizures and ischemia.
Recent studies have suggested that as many 20 percent of patients with autism have markers for mitochondrial disease, (Shoffner, J. the 60th Annual American Academy of Neurology meeting in Chicago, Apr. 12-19, 2008; Poling, J S et al J. Child Neurol. 2008, 21(2) 170-2; and Rossignol et al., Am. J. Biochem. & Biotech. (2008) 4, 208-217.). Some cases of autism have been associated with several different organic conditions, including bioenergetic metabolism deficiency suggested by the detection of high lactate levels in some patients (Coleman M. et al, Autism and Lactic Acidosis, J. Autism Dev. Disord., (1985) 15: 1-8; Laszlo et al Serum serotonin, lactate and pyruvate levels in infantile autistic children. Clin. Chim. Acta (1994) 229:205-207; and Chugani et al., Evidence of altered energy metabolism in autistic children, Progr. Neuropsychopharmacol. Biol. Psychiat., (1999) 23:635-641) and by nuclear magnetic resonance imagining as well as positron emission tomography scanning which documented abnormalities in brain metabolism. Although the mechanism of hyperlactacidemia remains unknown, a likely possibility involves mitochondrial oxidative phosphorylation dysfunction in neuronal cells. A small subset of autistic patients diagnosed with deficiencies in complex I or III of the respiratory chain have been reported in the literature (see Oliveira, G., Developmental Medicine & Child Neurology (2005) 47 185-189; and Filipek, P A et al., Journal of Autism and Developmental Disorders (2004) 34:615-623.) However, in many of the cases of autism where there is some evidence of mitochondrial dysfunction, there is an absence of the classic features associated with mitochondrial disease, such as mitochondrial pathology in muscle biopsy (see Rossignol, D. A. et al., Am J. Biochem. & Biotech, (2008) 4 (2) 208-217).
The diseases above appear to be caused by defects in Complex I of the respiratory chain. Electron transfer from Complex I to the remainder of the respiratory chain is mediated by the compound coenzyme Q (also known as Ubiquinone). Oxidized coenzyme Q (CoQox or Ubiquinone) is reduced by Complex I to reduced coenzyme Q (CoQred or Ubiquinol). The reduced coenzyme Q then transfers its electrons to Complex III of the respiratory chain (skipping over complex II), where it is re-oxidized to CoQox (Ubiquinone). CoQox can then participate in further iterations of electron transfer.
Very few treatments are available for patients suffering from these diseases. Recently, the compound Idebenone has been proposed for treatment of Friedreich's ataxia. While the clinical effects of Idebenone have been relatively modest, the complications of mitochondrial diseases can be so severe that even marginally useful therapies are preferable to the untreated course of the disease. Another compound, MitoQ, has been proposed for treating mitochondrial disorders (see U.S. Pat. No. 7,179,928); clinical results for MitoQ have not yet been reported. For KSS, administration of coenzyme Q10 (CoQ10) and vitamin supplements, have shown only transient beneficial effects in individual cases.
1,4-Benzoquinones with aryl substitution have been described in international patent publication WO 2008/002641 as selective inhibitors of protein tyrosine phosphatases to treat neoplastic disorders, but this publication does not specifically disclose 2-(3-hydroxy-3-methylbutyl)-6-(het)aryl-p-quinone or 2-(3-hydroxy-3-methylbutyl)-3-(het)aryl-p-quinone derivatives, nor the use of the compounds of this invention for the treatment of mitochondrial diseases.
The ability to adjust biological production of energy has applications beyond the diseases described above. Various other disorders can result in suboptimal levels of energy biomarkers (sometimes also referred to as indicators of energetic function), such as ATP levels. Treatments for these disorders are also needed, in order to modulate one or more energy biomarkers to improve the health of the patient. In other applications, it can be desirable to modulate certain energy biomarkers away from their normal values in an individual that is not suffering from disease. For example, if an individual is undergoing an extremely strenuous undertaking, it can be desirable to raise the level of ATP in that individual.